Method to increase coupling ratio of source to floating gate in split-gate flash

ABSTRACT

A split-gate flash memory cell having a three-dimensional source capable of three-dimensional coupling with the floating gate of the cell, as well as a method of forming the same are provided. This is accomplished by first forming an isolation trench, lining it with a conformal oxide, then filling with an isolation oxide and then etching the latter to form a three-dimensional coupling region in the upper portion of the trench. A floating gate is next formed by first filling the three-dimensional region of the trench with polysilicon and etching it. The control gate is formed over the floating gate with an intervening inter-poly oxide. The floating gate forms legs extending into the three-dimensional coupling region of the trench thereby providing a three-dimensional coupling with the source which also assumes a three-dimensional region. The leg or the side-wall of the floating gate forming the third dimension provides the extra area through which coupling between the source and the floating gate is increased. In this manner, a higher coupling ratio is achieved without an increase in the cell size while at the same time alleviating the punchthrough and junction break-down of source region by sharing gate voltage along the side-wall.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the manufacturing of semiconductormemories, and in particular, directed to a split-gate flash memoryhaving an increased coupling ratio of source to floating gate and to amethod of forming of the same.

(2) Description of the Related Art

The degree of coupling between a source and floating gate in asplit-gate flash memory is determined by the capacity of source implantlateral diffusion under the floating gate. It is desirable to increasethe coupling ratio because that results in improved programming speed.However, trying to increase coupling ratio by increasing the sourceimplant lateral diffusion causes the well-known problems ofpunch-through and junction breakdown. It is disclosed later in theembodiments of this invention a method of increasing the coupling ratioof a split-gate memory cell by extending into the trench isolationregion of the cell a three-dimensional source. Because of the additionalvertical wall area in the third dimension, the coupling is increased,and the performance of the cell improved.

A conventional split-gate flash memory device is characterized by itssplit-gate side (between the control gate and the drain) and thestacked-side (between the floating gate and the source) and by acoupling ratio between the floating gate and the source. As is known,the coupling ratio affects the program speed, that is, the larger thecoupling ratio, the faster is the programming speed, and is not a fixedvalue by virtue of the variability of the channel length and hence thatof the overlap between the floating gate and the source. Usually, ifchannel length is increased through greater lateral diffusion in thesource region, punchthrough occurs due to excessive current well belowthe threshold voltage. It is shown in the present invention that thecoupling ratio can be increased without increasing the channel length,but by incorporating side-wall coupling of the vertical wall in athree-dimensional source, thus alleviating the punchthrough and junctionbreak-down of source region by sharing gate voltage along the side-wall.

Over the years, numerous improvements in the performance as well as inthe size of memory devices have been made by varying the simple, basicone-transistor memory cell, which contains one transistor and onecapacitor. The variations consist of different methods of formingcapacitors, with single, double or triple layers of polysilicon, anddifferent materials for the word and bit lines. In general, memorydevices include electrically erasable and electrically programmableread-only memories (EEPROMs) of flash electrically erasable andelectrically programmable read-only memories (flash EEPROMs). Many typesof memory cells for EEPROMs or flash EEPROMs may have source and drainsregions that are aligned to a floating gate or aligned to spacers. Whenthe source and drain regions are aligned to the floating gate, a gateelectrode for a select transistor is separate from the control gateelectrode of the floating gate transistor. Separate select and controlgates increase the size of the memory cell. If the source and drainregions are aligned to a spacer formed after the floating gate isformed, the floating gate typically does not overlie portions of thesource and drain regions. Programming and erasing performance isdegraded by the offset between the floating gate and source and drainregions.

Most conventional flash-EEPROM cells use a double-polysilicon (poly)structure of which the well known split-gate cell is shown in FIG. 1.Here, two MOS transistors share a source (25). Each transistor is formedon a semiconductor substrate (10) having a first doped region (20), asecond doped region (25), a channel region (23), a gate oxide (30), afloating gate (40), intergate dielectric layer (50) and control gate(60). Substrate (10) and channel region (23) have a first conductivitytype, and the first (20) and second (25) doped regions have a secondconductivity type that is opposite the first conductivity type.

As seen in FIG. 1, the first doped region, (20), lies within thesubstrate. The second doped region, (25), also lies within substrate(10) and is spaced apart form the first doped region (20). Channelregion (23) lies within substrate (10) and between first (20) and second(25) doped regions. Gate oxide layer (30) overlies substrate (10).Floating gate (40), to which there is no direct electrical connection,and which overlies substrate (10), is separated from substrate (10) by athin layer of gate oxide (30) while control gate (60), to which there isdirect electrical connection, is generally positioned over the floatinggate with intergate oxide (50) therebetween.

In the structure shown in FIG. 1, control gate (60) overlaps the channelregion, (23 under the floating gate, (40). This structure is neededbecause when the cell is erased, it leaves a positive charge on thefloating gate. As a result, the channel under the floating gate becomesinverted. The series MOS transistor (formed by the control gate over thechannel region) is needed in order to prevent current flow from controlgate to floating gate. The length of the transistor, that is the overlapof the control gate over the channel region (23) determines the cellperformance. Furthermore, edges (41), (43) can affect the programming ofthe cell by the source size and hot electron injection through theintergate dielectric layer (50) at such edges. Hot electron injection isfurther affected by, what is called, gate bird's beak (43) that isformed in conventional cells. On the other hand, it will be known tothose skilled in the art that corners such as (41) can affect the sourcecoupling ratio also. Any such adverse effects attributable source sizecan be alleviated as disclosed later in the embodiments of thisinvention.

To program the transistor shown in FIG. 1, charge is transferred fromsubstrate (10) through gate oxide (30) and is stored on floating gate(40) of the transistor. The amount of charge is set to one of two levelsto indicate whether the cell has been programmed "on" of "off.""Reading" of the cell's state is accomplished by applying appropriatevoltages to the cell source (25) and drain (20), and to control gate(60), and then sensing the amount of charge on floating gate (40). Toerase the contents of the cell, the programming process is reversed,namely, charges are removed from the floating gate by transferring themback to the substrate through the gate oxide.

This programming and erasing of an EEPROM is accomplished electricallyand in-circuit by using Fowler-Nordheim tunneling as is well known inprior art. Basically, a sufficiently high voltage is applied to thecontrol gate and drain while the source is grounded to create a flow ofelectrons in the channel region in the substrate. Some of theseelectrons gain enough energy to transfer from the substrate to thefloating gate through the thin gate oxide layer by means ofFowler-Nordheim tunneling. The tunneling is achieved by raising thevoltage level on the control gate to a sufficiently high value of about12 volts. As the electronic charge builds up on the floating gate, theelectric field is reduced, which reduces the electron flow. When,finally, the high voltage is removed, the floating gate remains chargedto a value larger than the threshold voltage of a logic high that wouldturn it on. Thus, even when a logic high is applied to the control gate,the EEPROM remains off. Since tunneling process is reversible, thefloating gate can be erased by grounding the control gate and raisingthe drain voltage, thereby causing the stored charge on the floatinggate to flow back to the substrate.

In the conventional memory cell shown in FIG. 1, word lines (not shown)are connected to control gate (60) of the MOS transistor, while thelength of the MOS transistor itself is defined by the source (25) drain(20) n+ regions shown in the same Figure. As is well known by thoseskilled in the art, the transistor channel is defined by masking the n+regions. However, the channel length of the transistor varies dependingupon the alignment of the floating gate (40) with the source and drainregions. This introduces significant variations in cell performance fromdie to die and from wafer to wafer. Furthermore, the uncertainty in thefinal position of the n+ regions causes variations in the seriesresistance of the bit lines connected to those regions, and henceadditional variation in the cell performance.

In prior art, different methods for fabricating different split-gatememory cells are taught. In U.S. Pat. No. 5,495,441, Hong discloses asplit-gate flash memory cell having a vertical isolation gate and aprocess for making it. The memory cell has a floating gate transistorformed in a substrate having a channel extending underneath a floatinggate, and a vertical isolation transistor formed in the substrate havinga channel parallel to a trench holding a portion of a polysiliconcontrol gate and orthogonal to the channel of the floating gate. Inanother U.S. Pat. No. 5,414,287, Hong teaches a process for high densitysplit-gate memory cell for flash or EPROM. Silicon islands are formedfrom a silicon substrate implanted with a first conductivity-impartingdopant. A first dielectric layer surround the vertical surfaces of thesilicon islands, whereby the first dielectric layer is a gate oxide. Afirst conductive layer is formed over a portion of the vertical surfacesof the first dielectric layer, and acts as a floating gate for the highdensity split-gate memory cell. A source region is located in thesilicon substrate. A drain region is located in the top of the siliconislands. A second dielectric layer is formed over the top and sidesurfaces of the floating gate, and acts as an interpoly dielectric. Asecond conductive layer is formed over that remaining portion of thevertical surfaces of the first dielectric layer not covered by the firstconductive layer, and surrounds the second dielectric layer, whereby thesecond conductive layer is a control gate.

A different process for trench-isolated self-aligned split-gate EEPROMtransistor and memory array is described by Hazani in U.S. Pat. No.5,162,247. A still different method of manufacturing an EEPROM withtrench-isolated bitlines is taught by Gill, et al., in U.S. Pat. No.5,173,436. Here, an EEPROM cell is constructed using a floating-gatetransistor with or without a split gate. In this cell, the bitlines andsource/drain regions are buried beneath relatively thick silicon oxideand the floating gate extends over the thick silicon oxide. Programmingand erasing are accomplished by causing electrons to tunnel through theoxide in a tunnel window. The tunnel window has a thinner dielectricthan the remainder of the oxides under the floating gate to allowFowler-Nordheim tunneling. Trenches and ditches are used for electricalisolation between individual memory cells, allowing an increase in celldensity.

In the present invention, a method to increase the coupling ratio ofsource to floating gate is disclosed without increasing lateraldiffusion under the floating gate. This prevents punch-through andjunction breakdown problems. The increase in coupling ratio isaccomplished by providing a three-dimensional source extending intotrench isolation, whereby the vertical wall in the third dimensionprovides the extra area through which coupling between the source andthe floating gate is also increased. It will be appreciated by thoseskilled in the art that in this manner a higher coupling ratio isachieved without an increase in the cell size.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a method offorming a split-gate flash memory having an increased coupling ratiobetween source and the floating gate.

It is still another object of this invention to provide a method offorming a split-gate flash memory having a three-dimensional source inorder to increase the lateral coupling between the source and thefloating gate.

It is yet another object of the present invention to provide athree-dimensional coupling between the source and the floating gate of asplit-gate flash memory cell.

It is an overall object of this invention to provide a split-gate flashmemory cell of reduced size having three-dimensional source and henceincreased three-dimensional coupling between source and the floatinggate of the cell.

These objects are accomplished by providing a semiconductor substrate;forming a pad oxide layer over said substrate; forming a first nitridelayer over said pad oxide layer; forming and patterning a firstphotoresist layer over said first nitride layer to define active regionsin said substrate; forming a trench in said substrate by etching throughpatterns in said first photoresist layer and through said first nitridelayer; removing said first photoresist layer; forming a conformal liningon the inside walls of said trench; depositing isolation oxide insidesaid trench to form shallow trench isolation (STI); performingchemical-mechanical polishing of said substrate; removing said firstnitride layer; removing said pad oxide layer; forming and patterning asecond photoresist layer to define a three-dimensional coupling regionin said trench; etching back said oxide in said trench, thus forming athree-dimensional coupling region in the upper portion of said trench;removing said second photoresist layer; forming sacrificial oxide layerover said substrate; removing said sacrificial oxide layer; growing gateoxide layer over said substrate, including the sidewalls of saidthree-dimensional coupling region in said trench; forming firstpolysilicon layer over said substrate filling said three-dimensionalcoupling region in said upper portion of said trench; forming a secondnitride layer over said substrate; forming and patterning a thirdphotoresist layer over said second nitride layer to define cell area andform floating gate region in said substrate; etching through saidpatterning in said third photoresist layer into said second nitridelayer until portions of said first polysilicon layer are exposed;removing said third photoresist layer; using patterned said secondnitride layer as a mask, oxidizing said exposed portions of said firstpolysilicon layer to form poly-oxide; removing said second nitridelayer; using said poly-oxide as a hard mask, etching said firstpolysilicon layer to form floating gate; forming inter-poly oxide oversaid substrate; forming a second polysilicon layer over said inter-polyoxide; forming and patterning a fourth photoresist layer over saidsubstrate to define control gate region over said substrate; etchingthrough said patterning in said fourth photoresist layer into saidsecond polysilicon layer to form said control gate region; removing saidfourth photoresist layer; forming and patterning a fifth photoresistlayer over said substrate to define source implant region in saidsubstrate; performing source implantation; removing said fifthphotoresist layer; forming oxide spacers on side-walls of said controlgate; and forming drains of said split-gate flash memory cell.

These objects are further accomplished by providing a split-gate flashmemory cell having a floating gate with sidewalls extending into atrench, and thereby providing a three-dimensional coupling with thesource of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional split-gate typememory cell of prior art.

FIGS. 2A-2G are top views of a substrate showing the forming of asplit-gate flash memory cell of this invention without thethree-dimensional coupling between the source and the floating gate.

FIGS. 3A-3H correspond to the top views of FIGS. 2A-2G showing thecross-sections of the substrate of this invention without thethree-dimensional coupling between the source and the floating gate.

FIGS. 4A-4G are the top views and FIGS. 5A-5H are the correspondingcross-sectional views of the substrate of the preferred embodiment ofthis invention showing the forming of a split-gate flash memory cellwith a three-dimensional coupling capability, specifically:

FIG. 5A is a cross-sectional view of the substrate of FIG. 4A showingthe forming of the trench, according to this

FIG. 5B is a cross-sectional view of the substrate of FIG. 4B showingthe forming of the three-dimensional region in the upper portion of thetrench of this invention.

FIG. 5C is a cross-sectional view of the substrate of FIG. 4C showingthe photoresist defining of the cell and floating gate of thisinvention.

FIG. 5D is a cross-sectional view of the substrate of FIG. 4D showingthe forming of poly-oxide layer, according to this invention.

FIG. 5E is a cross-sectional view of the substrate of FIG. 4E showingthe forming of the floating gate and the extension of the "legs" of thesidewalls of the floating gate into the three-dimensional region of thetrench of this invention.

FIG. 5F is a cross-sectional view of the substrate of FIG. 4F showingthe forming of the second polysilicon layer over the floating gate ofthis invention.

FIG. 5G is a cross-sectional view of the substrate of FIG. 4G showingthe forming of the three-dimensional source around the three-dimensionalregion of the trench of this invention, and the resultingthree-dimensional coupling between the source and the floating gate,according to this invention.

FIG. 5H is a different cross-sectional view of the substrate of FIG. 4Gshowing the split-gate flash memory cell of this invention withthree-dimensional coupling capability.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, specifically to FIGS. FIGS. 4A-4H, andFIGS. 5A-5H, there is shown a preferred method of forming a split-gateflash memory having a three-dimensional source capable ofthree-dimensional coupling with the floating gate of the cell. FIGS.2A-2G and FIGS. 3A-3H show the present state of manufacturing asplit-gate flash memory cell. It will be apparent that the split-gate ofFIGS. FIGS. 2A-2G and FIGS. 3A-3H lack the three-dimensional region ofthe trench of the preferred embodiment where three-dimensional couplingcan take place between the floating gate and the source of thisinvention, as claimed later in the embodiments.

FIGS. 2A-2G show the top view of a semiconductor substrate on whichvarious process steps are performed. Corresponding to each top view, across-sectional view of the substrate representing the results of aparticular process step is shown in FIGS. 3A-3H.

Thus, FIG. 2A shows top view of a semiconductor substrate (100) whereactive regions (120) and trenches (130) have been defined. Across-sectional view of a trench is shown in FIG. 3A. First, a layer ofpad oxide (140), better seen in the cross-sectional view, is formed overthe substrate. Pad oxide layer may be formed by using chemical CVD SiO,or grown thermally.

Next, a first nitride layer (150) is formed over the pad oxide. Usually,nitride is formed by reacting dichlorosilane (SiCl₂ H₂) with ammonia(NH₃) in an LPCVD chamber. The active regions are next defined with aphotolithographic step and field regions grown, as is well known in theart. A first photoresist pattern is normally used to protect all areason which active devices will later be formed. The nitride layer is thendry etched, and the pad oxide may be etched by means of either a dry-orwet-chemical process. The etching is further carried into the substrateto form the trench (130) that is shown in FIG. 3A. The photoresist layeris next removed by oxygen plasma ashing and then the inside walls oftrench (130) is lined with an oxide layer (160) by thermal growth.Subsequently, the trench is filled with isolation oxide (170), thusforming shallow trench isolation (STI) as shown in FIG. 3A. Next, thesubstrate is subjected to chemical-mechanical polishing (CMP) afterwhich the nitride layer is removed. The removal of nitride layer can beaccomplished in a high-density-plasma (HDP) etcher. The pad oxide layer(140) underlying nitride layer (150) is also removed by using either adry- or -wet etch. Subsequently, a sacrificial oxide (not shown) isformed and removed, as is practiced in the art, in order to remove anyprocess related damage in the silicon substrate. The resulting structureshows oxide "caps" (170) that protrude above the STI (130) as seen inFIG. 3B, corresponding to the top view shown in FIG. 2B.

Next, floating gate oxide layer (175) is grown over the substrate,better seen in the cross-sectional view in FIG. 3C corresponding to thetop view of the substrate shown in FIG. 2C. Then, a first polysiliconlayer (180), later to be formed into a floating gate, is deposited overthe gate oxide layer. Polysilicon is formed through methods includingbut not limited to Low Pressure Chemical Vapor Deposition (LPCVD)methods, Chemical Vapor Deposition (CVD) methods and Physical VaporDeposition (CVD) sputtering methods employing suitable silicon sourcematerials. This is followed by forming a second nitride layer (190) overthe first polysilicon layer (180). The floating gates are next definedby patterning another photoresist layer (200) over the second nitridelayer. Second nitride layer is next etched through the patterns ofphotoresist layer (200) until portions of the fist polysilicon layer areexposed, after which the photoresist layer is removed. Using thepatterned second nitride layer as a mask, the exposed portions of thefirst polysilicon layer is wet oxidized to form poly-oxide (185) asshown in FIG. 3D. The second nitride layer is then removed. Thepoly-oxide layer (185) then serves as a hard mask to etch the firstpolysilicon layer to form floating gates (180) as shown in FIG. 3E. BothFIGS. 2D and 2E also show the top view of substrate (100) now havingfloating gates (180) formed thereon.

An interpoly oxide (210) is next formed over the floating gate as shownin FIG. 3F, and then a second polysilicon layer (220) is formed over theinterpoly oxide as shown both in the top view of the substrate in FIG.2F, as well as the cross-sectional view, FIG. 3F. Following the normalprocess steps of forming and patterning another photoresist layer todefine the control gate, and etching the second polysilicon layer toform the control gate, a structure is formed as shown in thecross-sectional view in FIG. 3G. Source region (105) shown in FIG. 3G isformed by conventional photoresist defining and the subsequent sourceimplantation. Top view in FIG. 2G better shows the drain regions (110)formed after the forming of oxide spacers (230). A cross-section at 3Hshows still better the source (105) and drain (110) regions. However, itis clear in FIG. 3G that the coupling, depicted by arrows, betweensource (105) and floating gate (180) is two-dimensional in the directionshown. In contrast, it will be shown in the preferred embodiment of thisinvention that the coupling can be affected three-dimensionally as shownin FIG. 5G.

The preferred embodiment shown in FIGS. 4A-4H and 5A-5H differ from thepresent state of the art in that the source region is formed about athree-dimensional region formed in the upper portion of a trench. Thus,in FIG. 4A, the top view of substrate (100), shows active regions (320),and regions (330) where shallow trench isolation (STI) are to be formed.First, a layer of pad oxide (340), better seen in the cross-sectionalview, FIG. 5A, is formed over the substrate. It is preferred that thepad oxide is grown thermally at a temperature range between about 850 to950° C., and to a thickness between about 100 to 250 Å.

Next, a first layer of nitride (350) is formed over the pad oxide.Preferably, the thickness of nitride layer (350) is between about 1200to 2500 Å, and that it is formed by reacting dichlorosilane (SiCl₂ H₂)with ammonia (NH₃) in an LPCVD at a temperature between about 750 to850° C.

The active regions are next defined with a photolithographic step (notshown) and field regions grown, as is conventionally performed. Aphotoresist pattern is normally used to protect all areas on whichactive devices will later be formed. The nitride layer is then dryetched, preferably using a recipe comprising SF₆ and O₂, and theunderlying pad oxide is also etched by means of CF₄, CHF₃ and O₂. Theetching is further carried into the substrate to form the shallow trench(330) that is shown in FIG. 5A by using a recipe comprising Cl₂, HBr,CHF₃ and O₂.

The first photoresist layer is next removed by oxygen plasma ashing andthen the inside walls of trench (330) is lined with an oxide layer (360)by thermal growth, preferably at a temperature between about 850 to 950°C. Subsequently, the trench is filled with isolation oxide (370), usingmethod of LPCVD (Low Pressure Chemical Vapor Deposition), or HDP (HighDensity Plasma), thus forming shallow trench isolation (STI) as shown inFIG. 5A. Next, the substrate is subjected to chemical-mechanicalpolishing (CMP). After CMP, nitride layer (350), as well as the padoxide layer (340) are removed, leaving nitride "caps" (370) protrudingover trenches (330). It is preferred that the removal of nitride isaccomplished in an HDP etcher with etch recipe comprising gases O₂, SF₆,CF₄ and He. The pad oxide layer (340) underlying nitride layer (350) isalso removed preferably by using wet or dry etch.

As a main feature and key aspect of the present invention, the next stepinvolves the forming of a three-dimensional coupling region withsidewalls reaching into the upper portion of trench (330) shown in FIG.5A. This is accomplished by forming a second photoresist layer (notshown) and patterning it with openings over trenches (330). Oxideprotrusions (370) are then etched, preferably with an etch recipecomprising 10:1 buffered oxide etch (BOE) or hydrofluoric, HF solution,to form coupling regions (335), as shown in FIG. 5B. The cross-section5B taken on the substrate is shown in the top view in FIG. 4B. After theforming of regions (335), the second photoresist layer is removed. Priorto the forming of the gate oxide layer, and as is known in the art, asacrificial oxide layer (not shown) is formed and removed in order toalso remove any damages that may have occurred in the silicon substrateduring the previous process steps.

Next, floating gate oxide layer (375) is grown over the substrate,including the sidewalls of the three-dimensional region (335), at atemperature between about 800 to 950° C., as shown in FIG. 5C.Subsequently, a first polysilicon layer (380), to serve as a floatinggate, is deposited over the substrate, filling regions (335). This isaccomplished preferably through a LPCVD method employing silane SiH₄ asa silicon source material at a temperature range between about 500 to650° C. Subsequently, a second nitride layer (390) is formed over thefirst polysilicon layer also by reacting dichlorosilane (SiCl₂ H₂) withammonia (NH₃) in an LPCVD at a temperature between about 700 to 850° C.This is followed by forming and patterning a third photoresist layer,(400), to define the floating gate cell regions. Photoresist layer (400)is also shown in the top view of FIG. 4C.

The second nitride layer (390) is next patterned by etching through thepatterns in photoresist layer (400) until portions of first polysiliconlayer (380) are exposed. The etching of nitride layer (390) isaccomplished with a recipe comprising CF₄, CHF₃, O₂, SF₆. Photoresistlayer is then removed by oxygen plasma ashing. The portions of the firstpolysilicon layer exposed at the bottom of openings in the secondnitride layer are wet-oxidized, forming poly-oxide (385), andsubsequently, nitride layer (390) is removed using another recipecomprising H₃ PO₄ solution.

Floating gate (380) is next formed by etching the first polysiliconlayer using poly-oxide (385) as hard-mask as shown in FIG. 5E. Theetching is accomplished with a recipe comprising HBr, O₂ and CL₂.Section 5E taken on substrate (100) is shown in the top view of FIG. 4E.It will be noted in FIG. 5E that the first polysilicon layer is etchedreaching into the three-dimensional coupling region (335). Thus,floating gate (380) forms "legs" (387) extending from its sidewalls,which surround the source region (305) that is to be formed at a laterstep.

An interpoly oxide (510) is next formed conformally over the sidewalland legs of the floating gate as shown in FIG. 5F. It is preferred thatthe interpoly oxide is formed through methods of thermal growth or hottemperature oxidation. Then, a second polysilicon layer (420) is formedusing the same process as for the first polysilicon layer over theinterpoly oxide as shown both in the top view of the substrate in FIG.4F, as well as the cross-sectional view, FIG. 5F. A fourth photoresistlayer (not shown) is used to define the control gate of the memory cell.The second polysilicon layer is etched to form control gate (420) byusing a recipe comprising HBr, O₂ and Cl₂. A fifth photoresist layer(also not shown) is used conventionally to define the source and sourceimplantation is accomplished by using phosphorus ions at a dosage levelbetween about 1×10¹⁵ to 1×10¹⁶ atoms/cm² and energy level between about20 to 60 KEV. Similarly, drain implantation is performed following theconventional forming of spacers, such as poly spacers (500) shown inFIG. 5H around the sidewalls of the control gate by depositing oxide andetching anisotropically. The drain implantation is accomplished by usingarsenic ions at a dosage level between about 1×10¹⁵ to 1×10¹⁶ atoms/cm²and energy level between about 20 to 60 KEV. Drain regions 40 are shownin the top view of FIG. (410), while the source region (405) is betterseen in the cross-sectional view of FIG. 5G. A different cross-sectionalview taken at section 5H shown in the top view of FIG. 4G is c shown inFIG. 5H where the split-gate cell of this invention is clearly seen.

A comparison of FIG. 5G with FIG. 3G show that the coupling betweensource (405) and floating gate (380) of this invention is affectedthree-dimensionally, as schematically depicted by vertical and lateralarrows in the same Figure, by virtue of the extension of the legs fromthe sidewall of the floating gate into the three-dimensional couplingregion (335) of trench (330). The leg or the side-wall of the floatinggate forming the third dimension provides the extra area through whichcoupling between the source and the floating gate is increased. In thismanner, a higher coupling ratio is achieved without an increase in thecell size while at the same time alleviating the punchthrough andjunction break-down of source region by sharing gate voltage along theside-wall.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of forming a split-gate flash memorycell having a three-dimensional source capable of three-dimensionalcoupling with the floating gate of the cell comprising the stepsof:providing a semiconductor substrate; forming a pad oxide layer oversaid substrate; forming a first nitride layer over said pad oxide layer;forming and patterning a first photoresist layer over said first nitridelayer to define active regions in said substrate; forming a trench insaid substrate by etching through patterns in said first photoresistlayer and through said first nitride layer; removing said firstphotoresist layer; forming a conformal lining on the inside walls ofsaid trench; depositing isolation oxide inside said trench to formshallow trench isolation (STI); performing chemical-mechanical polishingof said substrate; removing said first nitride layer; removing said padoxide layer; forming and patterning a second photoresist layer to definea three-dimensional coupling region in said trench; etching back saidoxide in said trench, thus forming a three-dimensional coupling sourceregion in the upper portion of said trench to allow three-dimensionalflow of electrons from said coupling region into said floating gate ofsaid cell; removing said second photoresist layer; forming sacrificialoxide layer over said substrate; removing said sacrificial oxide layer;growing gate oxide layer over said substrate, including the sidewalls ofsaid three-dimensional coupling region in said trench; forming firstpolysilicon layer over said substrate filling said three-dimensionalcoupling region in said upper portion of said trench; forming a secondnitride layer over said substrate; forming and patterning a thirdphotoresist layer over said second nitride layer to define cell area andform floating gate region in said substrate; etching through saidpatterning in said third photoresist layer into said second nitridelayer until portions of said first polysilicon layer are exposed;removing said third photoresist layer; using patterned said secondnitride layer as a mask, oxidizing said exposed portions of said firstpolysilicon layer to form poly-oxide; removing said second nitridelayer; using said poly-oxide as a hard mask, etching said firstpolysilicon layer to form floating gate; forming inter-poly oxide oversaid substrate; forming a second polysilicon layer over said inter-polyoxide; forming and patterning a fourth photoresist layer over saidsubstrate to define control gate region over said substrate; etchingthrough said patterning in said fourth photoresist layer into saidsecond polysilicon layer to form said control gate region; removing saidfourth photoresist layer; forming and patterning a fifth photoresistlayer over said substrate to define source implant region in saidsubstrate; performing source implantation; removing said fifthphotoresist layer; forming spacers on side-walls of said control gate;and forming drains of said split-gate flash memory cell.
 2. The methodof claim 1, wherein said semiconductor substrate is silicon.
 3. Themethod of claim 1, wherein said forming pad oxide layer is accomplishedby thermal growth at a temperature between about 850 to 950° C.
 4. Themethod of claim 1, wherein said pad oxide layer has a thickness betweenabout 100 to 250 angstroms (Å).
 5. The method of claim 1, wherein saidforming said first nitride layer over said pad oxide layer isaccomplished by CVD at a temperature between about 750 to 850° C. byreacting dichlorosilane (SiCl₂ H₂) with ammonia (NH₃).
 6. The method ofclaim 1, wherein the thickness of said first nitride layer is betweenabout 1200 to 2500 Å.
 7. The method of claim 1, wherein said firstphotoresist layer has a thickness between about 0.8 to 1.0 μm.
 8. Themethod of claim 1, wherein said forming a trench in said substrate byetching through patterns in said first photoresist layer is accomplishedby using recipe O₂, SF₆, CF₄ in etching through nitride layer, recipeCF₄, CHF₃ and O₂ through pad oxide layer, and recipe Cl₂, HBr, O₂ intosaid substrate.
 9. The method of claim 1, wherein said trench has adepth between about 3500 to 6500 Å.
 10. The method of claim 1, whereinsaid removing said first photoresist layer is accomplished by oxygenplasma ashing.
 11. The method of claim 1, wherein said conformal liningcomprises an oxide having a thickness between about 100 to 450 Å. 12.The method of claim 1, wherein said depositing said isolation oxideinside said trench to form shallow trench isolation (STI) isaccomplished by low pressure chemical vapor deposition-LPCVD.
 13. Themethod of claim 1, wherein said removing said first nitride layer isaccomplished with an etch recipe comprising gases O₂, SF₆, CF₄.
 14. Themethod of claim 1, wherein said removing said pad oxide layer isaccomplished by wet or dry etching.
 15. The method of claim 1, whereinsaid etching back said oxide in said trench to form saidthree-dimensional coupling region in said upper portion of said trenchis accomplished with an etch recipe comprising 10:1 buffer oxide etch(BOE) or hydrofluoric (HF) solution.
 16. The method of claim 15, whereinsaid three-dimensional coupling region has a depth between about 0.1 to0.2 μm.
 17. The method of claim 1, wherein said forming sacrificialoxide layer over said substrate is accomplished by thermal growth. 18.The method of claim 1, wherein said removing said sacrificial oxide isaccomplished with a recipe comprising HF solution.
 19. The method ofclaim 1, wherein said growing said gate oxide layer over said substrate,including the sidewalls of said three-dimensional coupling region insaid trench is accomplished by thermal growth at a temperature betweenabout 800 to 950° C.
 20. The method of claim 1, wherein said forming afirst polysilicon layer is accomplished with silicon source SiH₄ usingLPCVD at a temperature between about 500 to 650° C.
 21. The method ofclaim 1, wherein said first polysilicon layer has a thickness betweenabout 800 to 1500 Å.
 22. The method of claim 1, wherein said forming asecond nitride layer over said substrate is accomplished by CVD at atemperature between about 700 to 850° C. by reacting dichlorosilane(SiCl₂ H₂) with ammonia (NH₃).
 23. The method of claim 1, wherein saidetching through said patterning in said third photoresist layer intosaid second nitride layer until portions of said first polysilicon layeris exposed is accomplished with a recipe comprising CF₄, CHF₃, O₂, SF₆.24. The method of claim 1, wherein said oxidizing said exposed portionsof said first polysilicon layer to form said poly-oxide is accomplishedthrough thermal oxidation.
 25. The method of claim 1, wherein saidremoving said second nitride layer is accomplished with a recipecomprising H₃ PO₄ solution.
 26. The method of claim 1, wherein saidetching said first polysilicon layer using said poly-oxide as a hardmask is accomplished with a recipe comprising HBr, O₂ and Cl₂.
 27. Themethod of claim 1, wherein said inter-poly oxide layer comprisesoxide/nitride/oxide (ONO) having a thickness between about 100 to 300 Å.28. The method of claim 1, wherein said forming a second polysiliconlayer is accomplished with silicon source SiH₄ using LPCVD at atemperature between about 500 to 620° C.
 29. The method of claim 1,wherein said second polysilicon layer has a thickness between about 1000to 3000 Å.
 30. The method of claim 1, wherein said etching through saidpatterning in said fourth potoresist layer to form said control gateregion is accomplished with a recipe comprising HBr, O₂ and Cl₂.
 31. Themethod of claim 1, wherein said source implantation is accomplished withphosphorous at a dosage level between about 1×10¹⁵ to 1×10¹⁶ atoms/cm²and energy level between about 20 to 60 KEV.
 32. The method of claim 1,wherein said forming spacers on side-walls of said control gate isaccomplished by depositing and then anisotropically etching a layer ofpolysilicon.
 33. The method of claim 1, wherein said forming drains ofsaid split-gate flash memory cell is accomplished by implanting arsenicat a dosage level between about 1×10¹⁵ to 1×10¹⁶ atoms/cm² and energylevel between about 20 to 60 KEV.